• Each day we replace 10 11 red blood cells. These cells are killed in the spleen and subsequently replaced from stem cells. • Remember, stem cells are self renewing and can differentiate to a number of fates. • Blood cells (as well as intestinal crypt cells, epidermal cells and spermatocytes) are all at a steady state . • The critical stem cell in haematopoiesis is the pluripotent hematopoietic stem cell (HSC) • Produces all blood cells and lymphocytes of the body (generates different progenitor cells) whose potency is restricted to certain lineages
Hematopoesis • As with vasculogenesis, there are two phases in early vertebrate blood development: the extraembryonic primitive phase (happens in the early embryo) and the embryonic definitive phase (happens in late embryo/faetus and continues to the adult stage) • Phases differ in sites of blood cell production, timing and morphology of the cells produced as well as in the types of globin genes expressed in the red blood cells. • There are also two populations of hematopoietic stem cells , which correspond to the two phases Transient primitive phase is used to provide the young embryo with its initial blood through a capillary network starting within the yolk (blood islands) – yolk sack derived stem cells During definitive phase of blood development lateral plate splanchnopleurae near aorta produce definitive hematopoietic stem cell which will differentiate into more cell types as well as the stem cells needed for the entire life span
Nature Immunology 9, 129 - 136 (2008) There are two phases in vertebrate blood development: the primitive and the definitive phase. Remember: the fully developed organ consisting of trophoblast tissue and blood vessel containing mesoderm is the chorion . The chorion fuses with the uterine wall to create the placenta .
Blood cell mapping • In chick: first primitive blood cells , extraembryonic, are seen in the blood islands formed in the area opaca, in the posterior marginal zone, close to origin of secondary hypoblast • They can produce all blood cells except lymphocytes and they start producing red blood cells early in development. • First definitive blood stem cells originate from the mesoderm surrounding the aorta. • More specifically, they originate from the visceral lateral plate mesoderm near the aorta which has been called the a orta- g onad- m esonephros (AGM) . Aorta of the 3-day old chick embryo; arrows show hematopoietic cells in the AGM region
Nature Reviews Immunology 2, 593-604 (August 2002) A ) Early mouse embryo, showing the location of the aorta, gonads and mesonephros, as well as the area that has haemogenic activity (red outline). b) Transverse section through the embryo, showing the internal structure of the embryo at the level of the AGM.
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